The present invention relates to a fastening bolt comprising a screw and a nut, equipped with a safety device preventing the nut from being loosened on the screw.
In aeronautics, for example, for fasteners attaching motors to aircraft structures, it is necessary to give the fasteners a high level of safety given the risks of loosening, despite the heavy vibrations they are subject to. To that end, the tension imparted to a nut when it is tightened on a screw must be preserved in all circumstances, including after a period of use during which said fasteners may have been removed and then reinstalled multiple times.
It is known to use slotted nuts combined with screws that have rod holes, such as those described in document FR2955632.
In such a device, the nut comprises a threaded skirt that has undergone locking by elliptical deformation, meaning that the initially circular cross-section of the skirt has been flattened slightly. When tightened onto the threads of a circular cross-section screw, the skirt is forced to assume such a circular shape, which imparts a tension to the threads that impedes the loosening of the nut.
Furthermore, the skirt has an end forming slots, in which a rod housed in a hole passing through one end of the screw is fitted. The presence of the rod also prevents the nut from rotating around the screw.
Such devices, however, have a major risk of getting stuck due to the interference between the nut and the screw created by the deformation of the nut. This is because the slots of the nut or threaded skirt, deformed by the locking effect, might penetrate into the outlet bevels of the rod holes, when the nut is installed on the screw. The risk is present in steel screw/nut assemblies, and even more so for screw/nut assemblies in which at least one is made of titanium, for which the locking phenomenon is increased due to its poor tribological properties.
Furthermore, vibration tests have shown cases of fragile breakage of slots of existing fasteners.
Additionally, the practice has shown that in such devices, the elliptical deformation initially imposed on the threaded skirt exhibits a strong tendency to decrease after the fastener is removed and reinstalled several times, such that a substantial decline in the locking torque normally caused by that deformation is observed. This is because for some lock nuts of the prior art, the first installation of the nut on the screw causes a sizable reverse deformation of said nut towards a circular cross-section. When the nut is removed and the reinstalled, that plastic deformation causes a sizable decline in the locking torque.
In order to preserve the principle of dual security against unwanted loosening, despite the effect of removing and reinstalling the fasteners, an alternative was therefore sought to the solution consisting of combining a traditional slotted nut and a screw comprising a rod hole.
A first option consists of having the screw support a slot and having the nut support the rod holes, as in the device described in document U.S. Pat. No. 1,099,510. This document describes a screw whose outer surface comprises a slope receiving a rod, said rod passing through holes supported by lateral flanks of an associated nut.
However, the device of document U.S. Pat. No. 1,099,510 is obviously ill-suited to aeronautical use, and its shape makes elliptical deformation locking impossible. Furthermore, it does not solve the problem caused by being installed and removed in succession.